1. Field of the Invention
This invention relates to projection lens systems for use in projection televisions and, in particular, to high performance projection lens systems which 1) have a wide field of view, 2) have a high numerical aperture, and 3) are thermally-stabilized so that the focus of the system does not substantially change between room temperature and the system""s operating temperature.
2. Description of the Prior Art
Projection lens systems for CRT projection televisions have undergone continuing development during the past fifteen years or so. As a result, many of today""s CRT projection televisions are equipped with fast lens systems which have f-numbers of xcx9cf/1 and which provide semi-fields of view of 25xc2x0 and wider.
As is well-known, color images are obtained for television systems of this type by combining images from three color CRTs, i.e., a red CRT, a green CRT, and a blue CRT. Since the emission spectra of the CRTs are relatively narrow, for many applications, lens systems uncorrected for color can be used.
When the field of view does not exceed a half-angle of about 25xc2x0, the lens system may consist of just three components and still provide a sufficiently high level of image quality. A typical configuration has a weak aspherical element on the image side of the lens system, followed by a strong positive power element, followed in turn by a strong negative element in close proximity to the CRT. See, for example, Betensky, U.S. Pat. Nos. 4,300,817, 4,348,081, and 4,526,442.
For this configuration, the aspherical first element provides most of the correction of spherical aberration and other aperture dependent aberrations, the positive element in combination with the relative position of the aperture stop of the lens system allows for the astigmatism to be corrected, and the negative element close to CRT provides correction for the field curvature of the lens.
When the focal length of the lens system is shortened to make the projection television more compact, the field coverage of the lens system must be increased. When the half-angle of view is increased to about 28xc2x0, a three element form generally cannot provide an appropriately high level of optical performance. To address this problem, a fourth element has been added between the strong positive and the strong negative elements of the three component configuration. See Betensky, U.S. Pat. No. 4,697,892, and Moskovich, U.S. Pat. Nos. 4,682,862, 4,755,028, and 4,776,681. This additional element usually does not have much optical power; however, it must have an aspherical surface to correct for aperture dependent off-axis aberrations like sagittal oblique spherical and coma. Four element configurations have been used effectively up to half-angles of 33xc2x0 to 35xc2x0.
Five element configurations are also known in the art. See the Moskovich xe2x80x2862 and xe2x80x2681 patents, supra. In some systems, six element configurations have been used. See Moskovich, U.S. Pat. Nos. 5,296,967 and 5,329,363.
The performance of existing projection lenses for CRT projection televisions and, in particular, the widely used four element projection lenses, needs to be improved to meet the ever increasing demands on projection television sets. With more computer-generated data as well as with more information being shown around the periphery of the screen, the degradation of the image quality of the lens across the whole field of view must be minimized. At the same time, the speed of the lens must be kept at about f/1.0 and the fall-off of image brightness toward the edges of the screen must be minimized. In addition, the higher information content of the image makes the focus drift due to changes in the operating temperature of the TV set more noticeable, and thus the thermal stability of the optical performance of the projection lens becomes more important.
In view of the foregoing state of the art, it is an object of the present invention to provide improved projection lens systems for use in projection televisions. More particularly, it is an object of the invention to provide projection lens systems which (1) are capable of covering a semi-field of view of at least 33xc2x0 at a f-number of less than 1.2 for a long conjugate of infinite length, (2) are substantially resistant to thermal drift, and (3) provide an image quality suitable for the display of computer-generated data over substantially the entire field of the projected image. Further, it is an object of the invention to provide projection lens systems having the foregoing properties which can be manufactured at a reasonable cost.
To achieve the foregoing and other objects, the invention provides a projection lens system for use in combination with a cathode ray tube which from its long conjugate to its short conjugate consists of:
(a) a first lens element (L1) which has (i) a weak optical power and (ii) at least one aspherical surface;
(b) a second lens element (L2) which (i) has a positive focal length f2 and (ii) provides most of the optical power of the lens system, i.e., f2/f0 is less than 1.3 where f0 is the focal length of the projection lens system in combination with the CRT tube;
(c) a third lens element (L3) which has (i) a positive focal length f3 and (ii) at least one aspherical surface; and
(d) a fourth lens unit (U4) which is associated with the CRT during use of the lens system and (i) has a strong negative optical power and (ii) provides most of the correction of the field curvature of the lens system;
and which further consists of:
(e) a corrector lens element (CR) which is located adjacent to the second lens element and has (i) a weak optical power and (ii) at least one aspherical surface;
wherein:
2 less than f3/f2 less than 4,xe2x80x83xe2x80x83(1)
|fc/f3 | greater than 3, and (2)
max|z|/rca less than 0.15xe2x80x83xe2x80x83(3)
where fc is the focal length of the corrector lens element, max|z| is the maximum sag of the two surfaces of the corrector lens element, and rca is the clear aperture radius of the surface having the maximum sag.
As can be seen from relationships (1) and (2), the corrector lens element has a significantly longer focal length than the third lens element, and the third lens element has a significantly longer focal length than the second lens element, i.e., the power element. The corrector lens element is thus truly a corrector in that it has essentially no power compared to the other components of the system.
Relationship (3) relates to the manufacturability of the lens systems of the invention and, in particular, to the manufacturability of the corrector lens element. To simplify the process of manufacturing of plastic elements, and, thereby, to improve the ability to make the elements more accurately at a competitive price, it is desirable to have a general element shape which is approximately flat. If an element does not have much optical power but has a strong meniscus shape it is, generally speaking, more difficult to manufacture than an element of the same power but with flatter surfaces. The clear aperture of the corrector element located adjacent to the second lens element is determined by a full axial beam. Therefore, this element is more sensitive to manufacturing tolerances and must be made well. To make this task easier the overall element shape should have a max|z|/rca ratio which is less than 0.15, preferably less than 0.14, and most preferably less than 0.13.
In certain preferred embodiments of the invention, the lens system also satisfies the relationship:
tc/t2 less than 0.35,xe2x80x83xe2x80x83(4)
where tc is the distance between the corrector lens element and the second lens element and t2 is the thickness of the second lens element. Preferably, the tc/t2 ratio is less than 0.15.
The term xe2x80x9cweakxe2x80x9d is used herein to describe an element whose focal length has a magnitude which is at least about 5 times f0, and the term xe2x80x9cstrongxe2x80x9d is used to describe an element or unit whose focal length has a magnitude which is less than about 3 times f0.